["Addiction, EarlyView. ", "\nAbstract\n\nBackground and Aims\nCannabis is among the most widely used psychoactive substances globally and is often consumed alongside tobacco. Cannabis use has been associated with impairments in attention, learning, and memory, whereas nicotine can acutely enhance, but chronically impair, certain cognitive functions. Most studies examine cannabis in isolation, leaving the cognitive impact of tobacco co‐use unclear. This study aimed to estimate differences in cognitive performance between individuals with cannabis use disorder (CUD) and healthy controls, to determine whether cognition relates to heaviness of cannabis use or CUD symptom severity, to test moderation by daily tobacco use, and to assess whether baseline cognition was associated with cannabis outcomes one year later.\n\n\nDesign\nCross‐sectional and one‐year longitudinal study.\n\n\nSetting\nThe Netherlands and Texas, USA.\n\n\nParticipants\nA total of 231 participants aged 18–30 participated: 130 with CUD (57.7% male) and 101 controls (43.6% male).\n\n\nMeasurements\nParticipants completed tasks assessing interference control, attentional bias, sustained attention, executive functions, emotion recognition, delayed recall memory, working memory, and intelligence quotient (IQ). Primary outcomes were cognitive task scores; predictors included heaviness of cannabis use, CUD symptom severity, and daily tobacco use.\n\n\nFindings\nCompared with controls, individuals with CUD performed worse on interference control (rank biserial correlation [rrank] = 0.080, p < 0.001), immediate recall memory (rrank = 0.089, p = 0.009), delayed recall memory (rrank = 0.090, p = 0.013), executive functions (rrank = 0.089, p = 0.016), and estimated IQ (rrank = 0.081, p < 0.001). Within the CUD group, cognitive performance was unrelated to heaviness of use or CUD severity. Before correction, daily tobacco use moderated the link between CUD severity and working memory (p = 0.011, unstandardized beta [B] = −1.83), with poorer performance observed only among non‐tobacco users. Lower attentional bias (p = 0.027, B = −1.78) and sustained attention (p = 0.023, B = −27.88) were modestly associated with greater CUD severity at one‐year follow‐up.\n\n\nConclusions\nCannabis use disorder (CUD) appears to be associated with deficits in several cognitive domains independent of use intensity or severity. Tobacco and cannabis co‐use appears to be related to relatively better working memory. Attention‐related cognition appears to have limited associations with later CUD outcomes.\n\n"]